Cut the loaf of bread horizontally and remove most of the ‘insides’ forming a shell. Layer the swiss cheese on the bottom half of the bread, letting some hang over the sides of the loaf. Top with the mozzarella cheese. Sauté the mushrooms, shallots and garlic in olive oil over medium heat for 5-6 minutes, or until they begin to soften. Season to taste with salt and pepper. Place the mushroom mixture atop the cheeses, pushing down lightly to pack all of the deliciousness inside. Sprinkle the chives, poppy seeds, and paprika evenly atop the bread. In a small bowl, combine the butter, mustard and lemon juice and stir to combine. Pour the mixture over the open ends of the bread. Place the top of the bread back on and wrap loosely in aluminum foil. Place the bread on a baking sheet and bake at 350 degrees for 35-40 minutes or until golden and melty. Remove from oven and let rest on a cutting board for 10 minutes. Slice and serve.